﻿using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace Demo01
{
    class Program
    {
        static void Main(string[] args)
        {
            Console.WriteLine("Hello World");

            double a = 3.14;
            double b;
            Console.WriteLine("请输入圆的半径值");
            double r = double.Parse(Console.ReadLine());
            b = r * r * a;
            Console.WriteLine("圆的面积为" + b);
            Console.ReadKey();
        }
    }
}
